The invention relates to a ball draw-off apparatus for shaft prostheses with balls which can be placed onto the prosthesis neck.
In re-operations of hip joints it occurs that a femur shaft is still anchored so well that a new shaft is not necessary. In such a case it is sufficient to remove the joint ball from the prosthesis shaft and to place on a new joint ball, which for example is combined with a completely new acetabulum.
Up to now joint balls, which are placed on with a conical press fit on a femur shaft, were grasped by an auxiliary tool and removed through cautious hammering out with a sliding hammer. This hammering out at a shaft which is intact and anchored in the bone represents an unnecessary risk for a loosening of the shaft, since the adhesion of the ball on the shaft can not be predicted exactly.
The object of the invention is to create simple instruments which ensure a careful removal of the ball. This is achieved in that the apparatus has two rotary bodies which are rotatably supported in a housing, which can in each case be rotated with a lever arm and which lie with their axes of rotation in a common plane and form an open gap S in the projection onto the plane F for the introduction of the prosthesis neck, with the rotary bodies having a profile which with increasing rotation decreases the gap S for clamping the prosthesis neck and causes an increase of the distance of its upper edge from the plane F in order to produce a draw-off force P at the placed on ball directly or indirectly via an intermediate body.
The advantage of this apparatus lies in that it can be used for the most varied embodiments of prosthesis shafts, since a clamping is made at the prosthesis neck which is so strong that a required draw-off force can be produced with this neck support, with the clamping force and the draw-off force being produced by two oppositely moving lever arms, the bending torques of which compensate in regard to the clamping position when equal and opposite forces act at the levers. The apparatus consists of few parts and is simple to sterilize. Experiments have shown that marks arise at the prosthesis necks with the production of the clamping force which reflect the high surface pressing. In this the profile at the rotary bodies is designed such that the product of the clamping force times the friction is always somewhat greater than the required draw-off force. In the event of very large balls the upper edge of the clamping body can grip on directly at the socket of the ball with the rotation of the rotary bodies. In most cases however an intermediate body is preferred which transmits the draw-off forces from the upper edge of the rotary body to the ball.
It is however also possible in accordance with independent claim 2 to transfer the rolling away movement of the profile of the rotary body directly to the housing in the form of a stroke and to draw off the ball with a correspondingly slit housing.
Further improvements of the invention result from the subordinate claims 3 to 14.
If the gap S between the rotary bodies which is provided for the reception of the prosthesis neck contracts in the shape of a V, prosthesis necks with diameters of different sizes can be moved in for drawing off the balls. This can be achieved by a slight conicalness of the rotary bodies in the event of parallel axes of rotation or through a V-shaped arrangement of the axes of rotation and rotary bodies which are cylindrical in the region of the gap S. The latter arrangement has the advantage that equal rotational-angle-dependent relationships for the clamping and the for the lifting of the upper edge arise independently of the diameter of the prosthesis neck. It is therefore sufficient to move up on the neck of the prosthesis with a fork-like intermediate body, to move in below this with the draw-off apparatus, to press both instruments in the direction towards the ball and to actuate the levers in order to cause a clamping and a drawing off at the same time. In short prosthesis necks a separate fork-like intermediate body is advantageous since its limbs can be designed to be very low when they lie transversely to the upper edge of the rotary bodies.
One can however also anchor a fork-like intermediate body movably at the housing, the moving-in slit of which extends parallel to the gap between the rotary bodies. This arrangement has the advantage that no third hand is required to hold the intermediate body and that the operator can draw off the ball without additional help.
If the rotary bodies are arranged in a mirror imaged manner and are connected to one another kinematically in a mirror imaged manner the upper edges of the rotary bodies rise simultaneously and there arises a symmetric stress.
The rotational-angle-dependent theoretical gap reduction of a rotary body is less than the stroke increase of its upper edge to the plane F. They form a ratio between 0.05 and 0.5.
The theoretical reduction of the gap width S which is predetermined by the profiles is adhered to only in a limited manner in that a slight plastic deformation is produced at the prosthesis neck, whereas a portion of the movement practically serves to drive the rotary body and its support apart elastically to such an extent that a required clamping force is produced.
For this reason it is desirable to support the rotary bodies at their outer side with support bodies which project from the housing as bending carriers.
The stroke of the upper edge of the rotary bodies should amount to more than 0.5 mm. A stroke of 3 mm is sufficient for the usual conical connection.
With the draw-off movement the lever arms reduce their spacing so that in the final phase both lever arms can be grasped by one hand in order to produce the maximum required draw-off force.